In today's strong interests of the environment, there have been increasing calls for a reduction in the sulfur content of diesel oil products in hydrodesulfurization methods. In Japan, studies of regulations covering a reduction in the sulfur content of diesel oil products had instigated in 1989. As a first step, the sulfur content in diesel oil products has been regulated to be limited to 0.2% by weight or less since April of 1992, and the possibility of reducing such a sulfur content to 0.05% by weight by 1997 has been studied as a second step.
In techniques using existing catalysts and conventional apparatus for hydrodesulfurization of diesel oils, a reduction in sulfur content to about 0.1 to 0.15% by weight is considered to be a maximum limit under severest reaction conditions. In existing apparatus, when a liquid space velocity is reduced to excess, a processing ability decreases thereby and it makes not only requiring an increase in size of installations but also causing an increase in side reactions which adversely affect the hue value and hue stability of a diesel oil product. Also, when a reaction temperature is elevated so as to increase catalytic activities, it causes side reactions and similar deleterious effects result in hue of the diesel oil products. Thus, it is desired to develop a catalyst and a hydrotreating process which functions under lower reaction conditions in severity by using existing apparatus for hydrodesulfurization of diesel oils.
Hydrotreating, i.e. hydrodesulfurization, is generally conducted under reaction conditions of a temperature in the range of from about 100.degree. to about 600.degree. C., preferably from 200.degree. to 450.degree. C., a pressure in the range of from about 10 to about 300 kg/cm.sup.2, preferably from 20 to 100 kg/cm.sup.2, a liquid space velocity in the range of from about 0.1 to about 10 hr.sup.-1, preferably from 0.5 to 6 hr.sup.-1 and a feeding rate in terms of a hydrogen/feed oil ratio in the range of from about 30 to 1500 Nm.sup.3 /KL, preferably from 50 to 500 Nm.sup.3 /KL. The hydrodesulfurization catalyst which is widely used comprises a metal compound of the Group VIB of the Periodic Table (generally molybdenum or tungsten) or the Group VIII of the Periodic Table (generally cobalt or nickel) supported preferably on a refractory carrier.
Hitherto, various proposals have been made for such hydrodesulfurization catalysts, carriers for catalysts, and methods for preparation and use thereof.
More particularly, JP-A-50-64190 (the term "JP-A" as used herein means an unexamined published Japanese Patent Application) discloses that the degree of desulfurization can be increased and also a reduction in desulfurization activities can be suppressed by adding a rare earth element (cerium or lanthanum) to a catalyst having different sizes of micropores and comprising a metal of the iron group or a metal of the Group VIB of the Periodic Table supported on a refractory carrier.
JP-A-52-5691 discloses a method for preparing a catalyst by refluxing a catalyst comprising a metal of the iron group or a metal of the Group VI of the Periodic Table supported on a refractory carrier in an aqueous solution of an alkali or alkaline earth metal, a rare earth metal or a transition metal of the Periodic Table, but does not specifically teach the type of the rare earth metal and the transition metal to be used.
JP-B-53-6113 (the term "JP-B" as used herein means an examined published Japanese Patent Publication) discloses a method for preparing a hydrodesulfurization catalyst by mixing and peptizing a metal compound of the Group VIB, a metal compound of the Group VIII of the Periodic Table and a refractory inorganic compound to form an extrudable dough, extruding the dough, drying and calcining the resulting dough, and impregnating the calcined material with a metal compound of the Group VIB and a metal compound of the Group VIII of the Periodic Table, followed by drying and calcining of the resulting material in an oxidation atmosphere.
JP-A-54-127406 discloses an improved catalyst for hydrodemetallization and hydrodesulfurization of a hydrocarbon oil, specifically a petroleum distillate such as a residual oil, especially, a catalyst suitable for hydrodesulfurization, and discloses a hydrodemetallization catalyst containing a metal of the iron group and a metal of the Group VIB of the Periodic Table and prepared by impregnating alumina with about 0.5 to about 7.0% by weight, preferably about 1.0 to about 6.0% by weight of an oxide of a rare earth element (RE.sub.2 O.sub.3), preliminary calcining the impregnated alumina at a temperature of from about 704.degree. to about 927.degree. C. to thereby compound alumina and the oxide of rare earth element, followed by an impregnation treatment thereof.
JP-A-60-255143 discloses a method for preparing a supported hydroconversion catalyst which comprises preparing an aqueous solution for impregnation having a pH value in the range of from 0.7 to 2.7 by mixing at east one of molybdenum or tungsten compounds, one of cobalt compounds and/or one of nickel compounds, phosphorus in a stabilizing amount of from 0.2 to 1.0 mol per mol of Mo or W, and an appropriate soluble amine compound in an amount of from 2 to 6% by weight based on the weight of a carrier, and impregnating an appropriate catalyst carrier with the above-prepared solution, followed by drying and calcining of the resulting complex material.
JP-A-61-138537 discloses a process for preparing a hydrodesulfurization catalyst which comprises dehydrating filter cake of amorphous alumina hydrate adjusted to a pH of 7.5 to 10.5 by using a filter press to increase a concentration of Al.sub.2 O.sub.3 to 28 to 35% by weight kneading the filter cake in a self-cleaning type kneader for a residence time of 10 seconds or more, simultaneously adding each of the metals of the Group VIB and Group VIII of the Periodic Table to the resulting dough in an amount of from 20 to 60% by weight (in terms of a metal) in the form of an aqueous solution of a water-soluble salt of the metal just before the inlet of the kneader, succeedingly kneading, the resultant molding the dough by extrusion from an extruder, drying and calcining the molded material, impregnating the calcined material with an aqueous ammonia solution of the remaining catalyst metals, and drying and calcining the impregnated material.
Further, JP-B-2-54142 discloses a process for preparing an alumina catalyst carrier which comprises reacting aluminum sulfate with sodium aluminate at a pH of 6.0 to 8.5 and at a temperature of from 50.degree. to 65.degree. C. to prepare a first aqueous slurry containing amorphous alumina hydrate, adding sodium aluminate to the aqueous slurry to prepare a second aqueous slurry having an Al.sub.2 O.sub.3 concentration of 7% by weight or more, separating amorphous alumina hydrate contained in the second aqueous slurry by filtration, washing the resulting filter cake successively with a dilute aqueous solution of ammonia, a dilute aqueous solution of nitric acid and again a dilute aqueous solution of ammonia to adjust the pH value of the filter cake to 7.5 to 10.5, dehydrating the filter cake with a filter press to increase the Al.sub.2 O concentration thereof to 28 to 35% by weight, thereafter kneading the filter cake in a self-cleaning type kneader for a residence time of 10 seconds or more, extruding and molding the resulting dough, followed by drying and calcining of the resulting molded material.
In order to comply with secondary regulations projected for implementation in 1997 in Japan, it is necessary to reduce the sulfur content in diesel oil products to 0.054 by weight or less, and therefore very slightly active sulfur compounds (polyaromatics sulfur compounds), e.g. present in the ultra deep desulfurization region of ultra low sulfurs which are conventionally not included in the target compounds must be hydrodesulfurized.
For achieving the above-described ultra deep hydrodesulfurization by using conventional desulfurization apparatus and hydrodesulfurization catalysts for diesel oil, the use of either an increased reaction temperature or a decreased liquid space velocity has been considered. However, when the former method is applied, catalytic activities and catalyst life tend to decrease, and also the hue value and hue stability of a diesel oil product is adversely affected thereby markedly to reduce the commercial value of the resulting diesel oil product. On the other hand, when the latter method is applied, it requires enlargement of an installation due to insufficient capacity of existing equipment thereby to make it almost impossible to apply the method. Thus, since the conventional hydrodesulfurization catalyst cannot be applied to ultra deep desulfurization, it is highly desired to develop a new desulfurization catalyst having higher activities and a longer life than those conventionally employed.
Also, with the conventional catalyst, there are problems in that a degree of coloring tends to increase when the sulfur contents of diesel oil is reduced too much, and the sulfur contents tends conversely to increase when a degree of coloring is lowered.